1. Field of the Invention
The present invention generally relates to a network system, a node device and a communication method. More particularly, the present invention relates to a node device for connection of at least a piece or unit of equipment terminal equipment, a network system which includes a channel (for example, optical wavelength) division multiplexed transmission line for connection of a plurality of the node devices, and a communication method for transmitting a packet through the node devices in the network system.
2. Related Background Art
In recent years, study and development have been made with respect to network systems each of which employs a plurality of channels for transmission, such as an optical wavelength multiplexed transmission line utilizing a wide range of optical wavelengths, since a high-speed and large-capacity network system, which includes terminal equipments connected to a plurality of node devices, is required, following an increase in speed of processing in each terminal equipment. Such network systems, node devices and communication methods are roughly classified into two types.
A first classification, as shown in FIG. 14, includes a network system which is composed of a plurality of node devices 89 for connection of a plurality of terminals 95 and 96 and an optical wavelength multiplexed transmission line 97 which involves plural wavelength channels and performs information transmission and reception by connecting the plurality of node devices 89.
In the first type of the network system of FIG. 14, a packet transmitted from the terminal equipment 95 and input to an input I/F unit 93 is wavelength-exchanged in an exchange unit 91 so as to be transmitted from one of a plurality of fixed wavelength transmission units 92 at a predetermined wavelength.
The packet is then output to the fixed wavelength transmission unit 92 and transmitted at the predetermined wavelength therefrom. Afterward, relay processings are made through relaying node devices which exist on the way to the node device connected to a destination terminal equipment to which the packet is addressed. In the relaying node device, the packet is wavelength-exchanged in the exchange unit 91 which exchanges the input wavelength of the packet by detecting the address of the packet.
Finally, the packet is received at a fixed wavelength reception unit 90 in the destination node device, the address of the packet is detected in the exchange unit 91 and the output destination of the packet is controlled by the exchange unit 91 so as to be output from an output I/F unit 94 to which the destination terminal equipment is connected. Accordingly, the packet is output from the proper output I/F unit 94 and received by the destination terminal equipment 96.
The exchange unit 91 of the node device is operative to route the packet to a desired terminal equipment connected to a desired node device, by controlling the exchange operation which selects an output port of the input packet from the fixed wavelength transmission units 92 and output I/F units 94.
A second classification includes a network system connected by a topological optical wavelength multiplexed transmission line, such as bus and star networks, which is generally called a transmission media shared-type system.
In such a network system, when each terminal equipment starts the transmission of a packet, the terminal equipment makes a request for use of the wavelength multiplexed transmission line to a server which manages the wavelength assigned to each of the terminal equipments. The terminal equipment is then assigned a usable wavelength from the server. This is a so-called demand assign method. The network system thus performs arbitration control, using the demand assign method, so as not to cause a wavelength conflictive or collision situation in which plural terminal equipments intend to use the same wavelength for transmission of the packet. As discussed above, in the network system of the second type, the transmission of a packet is executed by using the thus-assigned wavelength.
The above conventional systems, however, have some disadvantages as described below.
In the first type of conventional system, there arises a problem that cost of the node device increases because of a large-scale hardware of the exchange unit.
FIG. 15 is a first structural example of the conventional exchange unit 91 of the first type, illustrating a crossbar-type exchange unit which has N input terminals and N output terminals.
In FIG. 15, each of decoder units 98 reads the address portion of an input packet and instructs a control unit 102 on the output destination to which the packet should be output. At the same time, the decoder unit 98 sends out the packet to a next stage. FIFOs (First In First Out) 99 then store the input packets temporarily and output them to respective output lines one by one, in order of input, under the control of the control unit 102.
Input lines 100 supply switches 101 with the packet signals from the FIFOs 99. The switches 101 act as a switch-over as to whether to output the input packet signal to an output line 103 or not. The control unit 102 performs, in accordance with outputs from the decoders, read-out control of the FIFOs 99 as well as opening and closing control of the respective switches 101.
The output lines 103 supply the respective output destinations with the packet signals output from the respective switches 101.
FIG. 18 shows a packet to be exchanged in the packet exchange unit 91. In FIG. 18, an address portion 112 indicates a destination terminal equipment to which the packet is addressed, and a data portion 113 indicates data to be carried by the packet.
In the crossbar exchange apparatus, routing control is performed in the control unit 102 by controlling opening and closing action of the switch to which a desired destination is connected, so that the output destination can be changed. Arbitration control is also performed in the control unit 102 to determine which of plural inputs should be output, when a so-called output conflict occurs. In the output conflict, the plural inputs are intended to be output to the same output destination.
Under those controls, the exchange operation is carried out in the crossbar exchange. However, in the first example of the exchange unit 91 having N inputs and N outputs, N.times.N switches are needed, resulting in a very large-scale hardware.
Further, the first example of the exchange unit is required to connect to the same output line 103 N switch outputs of the switches 101, which connect between the plurality of input lines 100 and the plurality of output lines 103, so that wiring of the connection line will be prolonged, resulting in delay due to the long wiring, an increase in stray capacitance of the wiring, and the like. Accordingly, as the number of N inputs increases, it becomes harder to accelerate switching operation of the switch 101. In other words, the first type of the exchange unit is unsuitable for a high-speed exchange of input packet signals.
Furthermore, the first example of the exchange unit 91 is required to perform the arbitration control for each output destination by monitoring occurrence of the output conflict with respect to all the inputs. This also leads to an increase in the hardware scale of the control unit which needs to perform the above-discussed arbitration control.
Now, FIG. 16 shows a second structural example of the exchange unit 91, which is intended to overcome the problems in the first example of the exchange unit 91. This second type of the exchange unit 91 is constructed in a manner that 2.times.2 switches having two inputs and two outputs are connected in a multi-stage form. In FIG. 16, each of switches 104 is a 2.times.2 switch having two inputs and two outputs and performs both of functions; straight and cross. In the straight function, inputs are connected straight to outputs, while in the cross function inputs are connected crosswise to outputs. A set of 2.times.2 switches containing 12 pieces and connected to form a shuffle network constitutes an omega-type exchange unit having eight inputs and eight outputs.
FIG. 17 shows the internal structure of the 2.times.2 switch 104 having two inputs and two outputs as mentioned above.
In FIG. 17, a decoder I 105 and a decoder II 106 each read the address portion of an input packet and instruct the control unit on a corresponding output terminal to which the packet should be output. A FIFO (First In First Out) I 107 and a FIFO II 108 temporarily store the input packets and output them to selectors, in order of input, under the control of a control unit 111. The selector I 109 and the selector II 110 each select either of the FIFOs 107 and 108, which stores the packet signal to be output to the output destination, under the control of the control unit 111.
When the selector I 109 selects the FIFO I 107 and the selector II 110 selects the FIFO II 108, the switch is functionally in the straight state. Conversely, if the selector I 109 selects the FIFO II 108 and the selector II 110 selects the FIFO I 107, the switch is in the cross state.
In the second example of the exchange unit 91, the required number of the 2.times.2 switches 104 is NlogN-N/2 (the base of the log is 2), so that it can be smaller than that of the first example which includes the N.times.N switches. Nonetheless, there also arises another problem that the whole of the hardware becomes large because the 2.times.2 switches each require decoders, FIFOs, a control unit and selectors.
Further, the second example of the exchange unit 91 has the disadvantage that a so-called blocking phenomenon is likely to occur. In the blocking phenomenon, connection with a desired output destination can not be made, depending upon connection conditions of the other inputs, even if the connection has not been made from different inputs to a common output destination.
In FIG. 16, assuming that the input 5 is connected to the output destination 3, the 2.times.2 switch 104 on an upper left side will be set to the cross state. Under this condition, however, the input 1 can not be connected to the output destination 1 because the upper left 2.times.2 switch 104 needs to be set to the straight state, and thus, the blocking occurs.
As described in the foregoing, the first type of the conventional network system has the disadvantage that the node device increases in cost because of a large-scale hardware of the exchange unit which forms a main component of the node device.
On the other hand, the second type of the network system is typically constructed as shown in FIG. 19, with the following problems contained therein.
FIG. 19 shows an example of the second type of the conventional network system, which is constructed in a form in which a plurality of terminal equipments are connected through a bus-type network to a server which performs the usable wavelength assignment for each terminal equipment.
In FIG. 19, a bus-type wavelength multiplexed transmission line 114 is an optical fiber cable. A server 115 has a wavelength assignment function. Blocks 116 each indicate a terminal equipment. A power multiplexer and divider 117 guides an optical signal from a variable wavelength transmission unit 118 to the optical fiber 114 and branches an optical signal transmitted on the optical fiber 114 to supply the branched one to a fixed wavelength reception unit 119.
The variable wavelength transmission unit 118 contains a tunable laser diode (TLD) therein and is operative to convert a packet signal from a packet processing unit 120 into an optical signal having a predetermined wavelength under the control of a wavelength control unit 121 and supply it to the power multiplexer and divider 117. The fixed wavelength reception unit 119 is comprised of a filter, through which only an optical signal having a predetermined wavelength can be transmitted by cutting off signals at the other wavelengths, and a photodiode which is operative to convert the optical signal at the predetermined wavelength transmitted through the filter into an electric signal and output the electric signal therefrom.
The wavelengths to be transmitted through the filters of the fixed wavelength reception units 119 are assigned to the respective terminal equipments such that those assigned wavelengths are different among the terminal equipments. The wavelength control unit 121 controls the transmission wavelength from the variable wavelength transmission unit 118 to a desired wavelength. Finally, an assignment control unit 122 assigns a plurality of usable wavelengths to the respective terminal equipments in the network system and performs the arbitration control such that the wavelength conflict does not occur.
The conventional network system, as described above, necessarily has the arbitration function, by which the overlap of wavelengths in use of the respective variable wavelength transmission units 118 in the plural terminal equipments can be prevented, because the optical fiber 114, which is the bus-type wavelength multiplexed transmission line, is commonly used by the respective terminal equipments 116. Generally, a demand assign method is used to perform the arbitration control.
In this method, when transmitting a packet, the transmitting terminal equipment 116 first sets the transmission wavelength of its variable wavelength transmission unit 118 to an fixed wavelength acceptable to the server and sends the server a transmission request packet which clearly designates an address of a destination terminal.
On reception of the transmission request packet, the server 115 looks into whether an acceptable wavelength to the destination terminal equipment is available or not. The server then sets the transmission wavelength of its variable wavelength transmission unit 118 to an acceptable wavelength to the transmitting terminal equipment, which has sent the transmission request packet, and sends the transmitting terminal equipment a communication permission packet if available, or a communication non-permission packet if unavailable.
After the terminal equipment, from which the transmission request packet has been sent, receives either of the communication permission/non-permission packets, the transmitting terminal equipment sets the transmission wavelength of its variable wavelength transmission unit 118 to the acceptable wavelength to the addressed terminal equipment and transmits a desired packet, if the communication is found permissive. If not permissive, the transmitting terminal equipment waits for a predetermined interval of time, and re-sends the server the transmission request packet. This operation is repeated until the communication is permitted. The arbitration function is thus performed such that the overlap of transmission wavelengths from the respective variable wavelength transmission units of the plurality of terminal equipments can be prevented.
In the conventional network system of the second type, the filters in the respective terminal equipments are set to transmit therethrough only optical signals having different wavelengths, respectively, so that the wavelength of the optical signal incident on each photodiode can be specific as well. Accordingly, the transmission wavelength is changed at the tunable laser diode (TLD) of the transmitting terminal equipment, thereby realizing the routing function for sending a packet to a desired destination terminal equipment.
However, in the network system of the second type, it takes much time to conduct communications with the server for the arbitration, such as a transmission of the transmission request packet and a reception of the communication permission/non-permission packets.
Further, the arbitration control needs to be performed in the server, for all the wavelengths to be used on the network, and this puts too much load on the arbitration control unit of the server, so that arbitration itself takes much time, resulting in lowering of throughput in the network system. Furthermore, the wavelength control unit of each terminal equipment needs to adjust the transmission wavelength to a predetermined wavelength each time communication is conducted with the server and with the addressed receiving terminal equipment. This requires high-speed wavelength control, resulting in a large-scale hardware.
Considering the problems of the above-discussed conventional networks, the inventor of the present invention already made inventions on a node device and a network system as illustrated in FIGS. 20A and 20B, and filed the U.S. Patent application thereon with the U.S. Patent and Trademark Office on Dec. 28, 1995 (now U.S. Pat. No. 5,801,859).
In FIGS. 20A and 20B, a control unit 123 of the node device includes a buffer control unit for controlling the read-out of buffers and a wavelength control unit for controlling the transmission wavelengths of variable wavelength transmission units. An optical fiber 124 is used as an optical wavelength multiplexed transmission line. A power divider 125 divides an optical signal transmitted on the optical fiber 124 into eight portions and output them to eight fixed wavelength reception units.
The fixed wavelength reception units I 126 to VIII 133 are photodiodes and serve as fixed wavelength reception means. The fixed wavelength reception units I 126 to VIII 133 each receive only a packet which is transmitted as one of optical signals having wavelengths .lambda.1 to .lambda.8.
Separation-insertion units I 134 to VIII 141 serve as separation-insertion means, each of which is operative to separate a packet, which is to be transmitted to a sub-transmission line, out of a packet stream from each of the fixed wavelength reception units 126 to 133 and transmit it to the sub-transmission line, while it is operative to add a packet from the sub-transmission line to the packet stream from the fixed wavelength reception unit.
Buffers I 142 to VIII 149 serve as buffer means to temporarily store the packets from the separation-insertion units. Variable wavelength transmission units I 150 to VIII 157 are variable wavelength transmission means, which convert, under the control of the wavelength control unit, the packets from the buffers 142 to 149 into optical signals each having a predetermined wavelength out of wavelengths .lambda.1 to .lambda.8 and send them through a wavelength multiplexer 158 to the optical fiber 124.
The wavelength multiplexer 158 multiplexes the optical signals of wavelengths .lambda.1 to .lambda.8 which are sent from the eight variable wavelength transmission units 150 to 157, and supplies them to the optical fiber 124.
Sub-transmission lines I 159 to VIII 166 serve as packet transmission lines between the separation-insertion units 134 to 141 and terminal equipments I 167 to VIII 174. The terminal equipments I 167 to VIII 174 are connected to the sub-transmission lines I 159 to VIII 166, respectively. Each of the terminal equipments receives a packet output from each of the corresponding separation-insertion units 134 to 141, while it generates a packet to be transmitted to another terminal equipment and sends it through each of the sub-transmission lines 159 to 166 to each of the separation-insertion units 134 to 141.
FIG. 21 is a block diagram of a network system in which four node devices of FIGS. 20A and 20B are connected by optical fibers. Node devices 175 to 178 shown in FIGS. 20A and 20B are respectively connected to eight terminal equipments 167 to 174 through eight sub-transmission lines 159 to 166. Optical fibers 179 to 182 are each used as an optical wavelength multiplexed transmission line.
In the example illustrated in FIGS. 20A and 20B, a packet from the terminal equipment is inserted, in each of the separation-insertion units 134 to 141, into the packet stream from each of the fixed wavelength reception units 126 to 133. The packet is temporarily stored in each of the buffer units 142 to 149, and then sent out from each of the variable wavelength transmission units 150 to 157 as an optical signal at a predetermined wavelength. The packet is relayed in the node devices located on the way to a node device which is connected to an addressed destination terminal equipment. The packet is then converted into an optical signal at a wavelength, which can be received by one of the fixed wavelength reception units 126 to 133 for outputting the packet to one of the separation-insertion units 134 to 141 connected to the addressed sub-transmission line, and transmitted from one of the variable wavelength transmission units 150 to 157 in the node device upstream of the node device connected to the addressed destination terminal equipment. The packet is finally received by the fixed wavelength reception unit in this node device, then output from the separation-insertion unit therein to the sub-transmission line and received by the addressed terminal equipment.